Weather, planting dates affect rust

Allison Tally says she was indeed surprised when USDA confirmed last month that the first case of Asian soybean rust in the United States had been found in a soybean field near Baton Rouge, La.

“I expected it might get here maybe next year or the year after,” says Tally, a plant pathologist who had predicted in a presentation just a few days before that Asian soybean rust might not arrive in this country for two or three years. “I thought it would be in Central America first and then here.”

Tally, who has made several trips to Brazil to monitor the spread of the disease in that country, says she's not sure where Hurricane Ivan — the storm most observers expect was responsible — picked up the spores for the disease. Some reports had Asian soybean rust in Columbia in August.

“But if it had to hit, this was a great time for it,” said Tally. “It gives us time for educating farmers about the disease before we begin another growing season.”

Following the initial discovery by an LSU AgCenter plant pathologist in Baton Rouge, the disease has now been confirmed in soybean fields in Alabama, Arkansas, Florida, Georgia, Mississippi, Tennessee and Missouri and on kudzu in Florida.

“It's troubling that they found soybean rust on kudzu in Florida,” says Tally, technical brand manager, fungicides, for Syngenta Crop Protection. “If the kudzu leaves remain green in that area, the disease will be more likely to over-winter.”

If the latter occurs in Florida or in South Louisiana, the chances of it reaching the Midwest or other parts of the Soybean Belt in 2005 are much improved.

“The disease spread over a 2,000-mile area of Brazil in one year, so it could move on the prevailing winds from the southern United States to the Midwest in one season if conditions are right,” says Tally.

Asian soybean rust or Phakopsora pachyrihizi originated in Japan in 1902. Initially, it spread very slowly, moving to China, Southeast Asia and Australia.

In 1996, it traveled on wind currents from Asia to Africa. It appeared first in Uganda and then in Zimbabwe and South Africa.

Scientists first detected the disease in a small area along the border between Paraguay and Brazil in the 2001-02 growing season (the seasons are reversed in the Southern Hemisphere.) By the end of the 2003-04 season, it could be found in all the soybean growing areas of Brazil and in Bolivia and Argentina.

“It probably traveled to Brazil on the trade winds from Africa,” says Tally. “No one brought it into the country — the spores were just carried on the trade winds.”

Soybean rust symptoms generally start with a yellowing of the leaves in the lower canopy of the plant.

As the disease develops, it produces tan and reddish brown lesions that, when mature, consist of small pustules.

“In Brazil, researchers have learned that the disease comes in fast and can defoliate soybean plants in a matter of days,” says Tally.

“It can infect plants in the vegetative stage but does not appear to progress to the middle and upper leaves until the plants reach the reproductive stage.”

Scientists have observed yield losses of 80 percent due to the rapid defoliation of plants in Zimbabwe and South Africa where farmers were not accustomed to treating fields for diseases.

“Farmers in Brazil are used to spraying fungicides so the damage has not been as severe,” she said.

“Brazilian soybean losses have been in the more moderate range of 10 percent to 50 percent.”

What makes soybean rust such a devastating disease over widespread areas is its prolific spore production.

The pustules in the lesions formed by the disease act as “spore pumps.” Researchers estimate that spore production can range as high as 4 trillion spores per acre once the lesions mature.

“If you achieve 98 percent control, you still have billions of spores left,” she says. “That could help explain why the disease has been confirmed over such a wide-spread area of the southeastern United States this fall.”

USDA's Economic Research Service calculated that Asian soybean rust cost Brazilian farmers $600 million during the 2002-03 season in yield losses and the expense of spraying about 80 percent of soybean area with fungicides. That figure could have exceeded $1 billion in 2003-04, according to ERS.

When the disease hits the United States with full force, ERS economists estimate the potential loss in the first year at $640 million to $1.3 billion and in subsequent years at $240 million to $2 billion, depending on the severity and extent of its spread.

It's possible that the early soybean planting systems used by farmers in the Mid-South could help lessen the impact of soybean rust.

“In Brazil, some farmers have been planting early to avoid the disease,” says Tally. “If the disease hits late, the losses don't seem to be as extensive as when it occurs during pod fill.

“The disease can be managed,” she said. “Some untreated fields in Zimbabwe did lose 80 percent of their yield to the disease. But early, effective fungicide programs kept losses in the 20 percent range. The key is acting early to minimize the impact of the disease.”

Tally says Syngenta is recommending that growers be aware of where soybean rust is found in the United States and the likelihood of it moving to their area so they can be prepared to spray.

“We're working with USDA's Animal Plant Health Inspection Service and the universities and other companies on a better system of forecasting,” she said. “If Louisiana has it when plants start growing next spring, where can it go? Can it reach farmers in Iowa?”

Roger Magarey, a researcher with USDA Agricultural Research Service at North Carolina State University, has developed a Soybean Rust Suitability Index that attempts to predict the probability of at least 15 days of suitable conditions for the disease between July 1 and Sept. 30.

A map generated by Magarey shows a high probability of the disease moving to Missouri, Iowa and southern Illinois if it becomes established in Louisiana.

“Now, if it's hot and dry, there may be no need to spray, based on what we've observed in Brazil. If conditions are wet and cool, a preventive spray may be warranted. We are saying that if you have the disease, then spray a fungicide. We're being conservative for now.”

She said she and other researchers are anxious to see how the fungus fares during the growing season in the United States. “It may like the climate in this country fine, and it may not. We won't know until we have experience during the primary growing season.”

Growers in the southern Brazilian state of Parana, where the climate is closely akin to that of the lower Midwest, have found that the disease is not as strong as in other parts of Brazil.

“We will also be watching the situation in Argentina closely this winter,” says Tally. “They got it late in their growing season last spring, and we're anxious to see how it develops so that we can learn more about how best to approach it.”